1. Field of the Invention
The present invention generally relates to an engine for an outboard motor, and more specifically, to a four-stroke engine that can include an oil return passage that communicates with an oil pan and a blow-by gas passage that can allow fluid communication between a crank chamber, a cam chamber and an interior of the oil pan.
2. Description of the Related Art
A conventional four-stroke engine for an outboard motor is described in an exemplary system shown in Japanese Patent Document No. JP-A-Hei 11-301592 (hereinafter “JP '592”). According to the system in JP '592, the engine is provided with a crankcase, a crankshaft supported by the crankcase so as to be rotatable around its axis, which extends generally in the vertical direction, a cylinder projecting from the crankcase, and an oil pan disposed below the crankcase.
In the system shown in JP '592, a lubrication device also is provided to lubricate the engine components with lubricating oil. In particular, the system includes a crank chamber oil-return passage, a cam chamber oil-return passage, and a gas passage. The crank chamber oil-return passage allows for the return of lubricating oil to the oil pan from an inner bottom of a crank chamber defined in the crankcase. The cam chamber oil-return passage allows for the return of the lubricating oil to the oil pan from the inner bottom of a cam chamber in the projecting end of the cylinder. Finally, the gas passage places the oil pan and the cam chamber in communication with each other.
When the engine is in operation, an oil pump is used to supply lubricating oil from the oil pan to portions of the engine that need to be lubricated. During lubrication, the lubricating oil tends to collect in the crank chamber and the cam chamber. The oil is recirculated to the interior of the oil pan through the crank chamber oil-return passage and the cam chamber oil-return passage so that it can be supplied again to the portions of the engine to be lubricated.
In addition, blow-by gas leaking from a combustion chamber into the crank chamber flows into the oil pan through the crank chamber oil-return passage, together with the lubricating oil. Then, the blow-by gas having been returned into the oil pan flows into the cam chamber through the gas passage and it is burned after being taken into the intake system. Here, as described above, lubricating oil tending to stay in the cam chamber is returned to the interior of the oil pan through the cam chamber oil-return passage. On the other hand, blow-by gas in the oil pan is forced to flow into the cam chamber through the gas passage formed in addition to the cam chamber oil-return passage.
The system of JP '592 tends to prevent interference between lubricating oil and blow-by gas. In particular, lubricating oil that has returned from the cam chamber to the oil pan does not interfere with blow-by gas that is forced toward the cam chamber from the oil pan. By eliminating any interference with the blow-by gas, the lubricating oil in the cam chamber is returned smoothly to the interior of the oil pan thereby preventing the accumulation of a large quantity of lubricating oil in the cam chamber.